This invention relates to fluorescent lamps and more particularly to a shield for the electron source of such lamps.
Fluorescent lamps are energy efficient light sources. An arc discharge occurring in the lamp generates actinic radiation, which causes fluorescence from a contained phosphor coating on the interior of the lamp. The electron source is generally a metal coil, usually tungsten, containing an electron emissive material. Two such coils are provided, one at either end of an elongated glass tube. During operation of the lamp it is not unusual for sublimation or sputtered products from the coils to plate out on the inside surface of the lamp adjacent the coils, causing undesired darkening of the glass and reduced light output.
To remedy this situation shields have been placed around the coils to catch the sublimation or sputtered products, which generally comprise tungsten, iron and nickel. Such shields, however, add to the cost and make assembly of the mount structure into the lamp envelope difficult since such shields, in the past, have substantially surrounded the coil, extending beyond a desirable perimeter. Further, the shields have been occasionally mounted to one or both of the lead-in wires for the electrode coil raising the possibility of short circuiting the coil in the event of misalignment In those instances wherein two separate shields have been mounted, one on each lead-in wire, the shields also function as an anode during one half of the cycle. Such shields, also, complicate the mounting and can lead to excess heating of the coil.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the operation of fluorescent lamps.
Yet another object of the invention is the enhancement of electrode coil shields.
These objects are accomplished, in one aspect of the invention, by the provision of a mount for a fluorescent lamp, which comprises a glass base, a longitudinal electrode coil mounted upon lead-in wires extending from the base, and a shield mounted upon the base. The shield comprises a pair of spaced apart shield plates, one on either side of the electrode coil and spaced therefrom. To avoid acting as an anode during half of the alternating current cycle, the shield plates are electrically isolated from the cathode. By electrically isolated is meant that the shields are not hard-wired into any part of the electrical circuit.
Use of these shield plates reduces the size of the shield and makes the insertion operation simpler and more accurate. Further, the shield plates do not act as an anode and thus allow for better controlled heating of the electrode coil.